In recent years, devices and applications involving LEDs (i.e., light emitting diodes) are gaining popularity. Such devices and applications range from light sources for general illumination, signs and signals, to display panels, televisions, etc. Regardless of the applications, LED driver circuits are used in supplying power to the LEDs.
An LED panel refers to a device that includes an array of LEDs that are connected together, or a device that includes plurality of sub-modules, each sub-module having such an LED array. LED panels usually employ arrays of LEDs of a single color or different colors. When individual LEDs are used in certain display applications, each LED usually represents a display pixel. An RGB LED unit refers to a cluster of three LEDs, namely, a red LED, a green LED, and a blue LED. When RGB LED units are used in certain display applications, each RGB LED unit may represent a display pixel. Surface mounted RGB LED units usually have four pins, one pin for each of the red, green, and blue LEDs and the fourth pin for either a common anode or a common cathode.
LED arrays are traditionally arranged in a common anode scan configurations, in which the anode of the LEDs are electronically connected to a power source via a switch element, while the cathodes of the LEDs are electronically connected to the output of current sink. In such a configuration, an N-MOS driver is often used as the current sink. An N-MOS is preferable over a P-MOS because N-MOS has a larger current capacity and a lower RDS (on) for a given design configuration.
In a common anode configuration, all RGB LEDs are connected to the same power supply and are supplied the same voltage. As is well-known in the art, the red LED forward voltage is significantly lower than that of green and blue LEDs. If the same supply voltage is used for the red, green, and blue LEDs, adjustments are required to match the forward voltages of individual LEDs, for example, by installing a bias resistor between the power supply and the LED. In that case, a significant amount of energy is dissipated as heat on the bias resistor. For example, if the supply voltage is 5 volts, since the forward voltage drop of a red LED is about 2.0 volts, approximately 60% of the energy is lost as heat on the bias resistor. Excessive heat dissipation wastes energy and complicates the design of driver circuitry because extra consideration needs to be given to increased demand of heat removal.
In addition, the display resolution increases when the size of the pixel pitch becomes smaller. The size of the pixel pitch is partially determined by the printed circuit board that holds a variety of components. Such components are, for example, a constant current driver, a decoder, power MOSFETs to control scan line switching, and bias resistors for some LEDs (such as red LEDs) to reduce LED driver operating voltage. In a design that these components are mounted on a PCB (printed circuit board) as discrete parts, the number of layers on a PCB needs to be increased. Such a design increases manufacturing cost as well as the difficulties in both noise reduction and pixel patch size reduction. In such a design with discrete parts, other problems may arise, such as timing control, parasitic capacitance, and ghost images, etc.
There is a need to design a highly integrated LED driving circuit with reduced cost, reduced heat dissipation, and reduced noise, which is capable of driving high resolution LED displays.